Introduction

The Dommel is a relatively small and sensitive river flowing through the city of Eindhoven (The Netherlands), receiving discharges from the 750,000 PE wastewater treatment plant (WWTP) of Eindhoven and over 200 combined sewer overflows (CSOs). In summer time, the WWTP effluent covers up to 50% of the base flow of the Dommel River, which does not yet meet the requirements of the European Union Water Framework Directive (WFD). Waterschap De Dommel, the utility responsible for this compliance, is conducting a research project in order to find the most cost-effective set of measures for meeting the WFD requirements by means of an integrated strategy for the urban wastewater system. The focus is on protection of the aquatic environment from oxygen dips and ammonia peaks caused by the combined discharges of the biologically treated WWTP effluent, a rain water buffer settling tank (RBT) at the WWTP and the CSOs within the Eindhoven area.

The reader is referred to Benedetti et al. (2012) and Langeveld et al. (2012) for details on the project, the model development and RTC strategies developments. The specific objectives of this contribution are the illustration of the latest modelling results of integrated solutions that include both RTC as well as capital-intensive measures. Moreover, the step-wise future implementation of the solutions in the real system is discussed.

Material and Methods

The project consists of several steps. Previous work focussed on the realisation of the integrated system model, the model analysis and the design of RTC. Here, we focus on the latest results, in particular regarding the evaluation of the step-wise implementation of the selected measures (Table 1).

Table 1 List of selected measures evaluated in scenarios.

Measure

Field of application/objective

RTC in the sewer system

Reduction of DO dips and NH4 peaks in river by better use of the available system capacity

Enhance nitrification process to reduce NH4 peak concentrations in river in WWF

Increase interceptor/pumping capacities

Reduce DO dips in river

Increase hydraulic capacity of biological treatment at WWTP

reduce NH4 peak concentrations and DO dips in river

The scenarios were first tested simulating the integrated model with three rain events with return periods of 5, 0.25 and 0.08 years as input. An ecological evaluation framework (Table 2) was then developed based on the local relationship between dissolved oxygen (DO) and NH4 and the presence of macro-invertebrates. The quality criteria are defined in terms of thresholds for low DO and high NH4 in several combinations of frequency and duration of exceedance, to be used in the evaluation of scenarios simulated for long periods (e.g. 10 years). This framework was applied to evaluate the water quality at six sections along the Dommel River, together with the calculation of capital and operational costs of measures.

Results and Conclusions

Some measures aim at a specific water quality issue, i.e. (1) DO depletion, (2) ammonia toxicity, and (3) summer average nutrients level, whereas others affect more than one issue. The first steps are the increase of the maximum hydraulic capacity of the WWTP to benefit more from RTC in the sewer system (lower risk of CSOs while storing water in the sewer), and a decrease of the NH4 set-point in the aeration control at the WWTP to reduce NH4 peaks (more responsive aeration system). Incremental steps were tested applying the measures listed in Table 1. Some results are shown in Figure 1 and in Table 2, where it can be seen that measures can be added until the performance is satisfactory. After that, to study more in detail the first actual implementations on the real system, the analysis focussed on (1) a modified use of the primary settlers, partly used as dry buffers (for NH4 peak shaving) and with dosage of coagulants in wet-weather (for COD reduction, i.e. increase of oxygen available for nitrification) and (2) RTC in the sewer system in the southern part of the catchment.

Figure 2 NH4 concentration at the river section 2 km downstream the WWTP effluent and most CSOs, with the 5-year return storm; detailed description of scenarios will be provided in the full paper.

Concerning cost evaluations, the reference scenario, consisting of conventional methods of solving water quality issues like uncoupling of paved area and building sewer storage facilities at CSOs, would require a yearly cost of about € 10.6M (designed to solve only DO problems in the river). The scenario fulfilling all the criteria would require about € 6.3M implying a yearly saving of € 4.3M (45%).

Table 2 Results of selected incremental scenarios with the ecological evaluation framework; scores from 1 (very good quality) to 5 (very bad quality) at the closing river section; details on scenarios will be provided in the full paper

It can be concluded that a large saving can result from the innovative integrated study and solutions, with the additional benefit of solving the NH4 peaks problem which was identified only during this project. A step-wise implementation of the selected measures will allow testing their actual impact, in order to better design the next step in the programme.